Photonic crystals have a structure in which dielectric materials with differing refractive indices are arranged with a periodicity that is close to the wavelength of light, and it is well known that they have the characterizing features of
(a) confining light by a photonic band gap;
(b) very large wavelength dispersion due to their special band structure; and
(c) group velocity anomalies of the propagated light.
Many optical devices that take advantage of these features have been proposed or are being studied.
The inventors of the present invention have studied the propagation of electromagnetic waves in one-dimensional and two-dimensional photonic crystals without periodicity in the propagation direction. Details are disclosed in JP 2002-236206A, for example. When a plane wave is coupled as the incident light to an end surface of a photonic crystal without periodicity in the propagation direction of the incident light, propagation light due to a plurality of photonic bands is generated, depending on the frequency of the incident light. Among these, propagation light waves due to bands that are not of the lowest order (in the following referred to as “higher-order band propagation light”) have the above-mentioned characteristics of “very large wavelength dispersion” and “group velocity anormalies”, so that they can be applied to various kinds of optical devices.
However, a portion of the energy of the incident light always is propagated as propagation light due to the lowest-order band (in the following referred to as “first band propagation light”). This first-band propagation light hardly displays the above-mentioned effects of “very large wavelength dispersion” and “group velocity anormalies” at all. Therefore, if the higher order band propagation light is utilized, this first-band propagation light merely will be loss. That is to say, the first-order propagation light not only considerably decreases the utilization efficiency of the incident-light energy of the device, but becomes also a cause of lowering the S/N ratio of the device due to stray light.
Now, research of the inventors has shown that propagation light within the photonic crystal that is due to a single higher order band propagation light can be obtained by coupling into the photonic crystal incident light that has been phase modulated at the same period as the end surface of the photonic crystal. Thus, the efficiency of various kinds of optical devices exploiting such features as wavelength dispersion or group velocity anomalies can be increased dramatically. The phase modulation of the incident light can be realized by a simple method, such as passing a plane wave through a phase grating, for example.
The electric field of a higher order band propagation light wave within a photonic crystal without periodicity in the propagation direction is split into two regions by two nodes within one period of the refractive index periodicity of the photonic crystal. There is also the characteristic that the phase of the propagation light within the various regions of the photonic crystal is shifted by half a period. In order to attain such a propagation light, the phase grating for phase modulation of the incident light has the same period in the same direction as the period of the photonic crystal.
Now, the period of the refractive index of the photonic crystal is shorter than the wavelength of ordinary light, so that also the period of the phase grating becomes shorter than the wavelength of the light, which leads to difficulties in the manufacturing process. For example, a method of cutting a portion of the photonic crystal away by forming grooves in it and splitting the photonic crystal into a waveguide portion and a phase grating portion is conceivable, but it is technically difficult to form narrow grooves having a high aspect ratio with high precision.
Moreover, in order to decrease the proportion of first band propagation light and increase the proportion of higher order band propagation light, the proportion between the zero-order diffraction light intensity and the ±1-order diffraction light intensity due to the phase grating as well as the phase needs to be adjusted, so that optimization design is required.